The benzoquinoid ansamycins have proven to be selectively cytotoxic in vitro to poorly differentiated solid tumors. Cytotoxicity does not correlate with alterations in protein tyrosine kinase activity or cellular phosphotyrosine content. Our focus in this project is now to determine the mechanism of selective cytotoxicty of these compounds. We have determined that all cytotoxic ansamycins have the property of binding to the heat shock protein hsp9O within 30 min of drug addition to cells. The sequelae of this protein binding are currently under investigation, but we can clearly show a disruptive effect on src activity, erbB2 kinase, and mutated p53. In screening a wide variety of ansamycin derivatives, we have determined that a very strong correlation exists between ability to bind hsp9O and cytotoxicity. Since hsp9O is involved in many intracellular functions, including glucocorticoid receptor function and raf kinase activity, the cytotoxic effects of the ansamycins probably involve disruption of specific hsp9O/protein interactions which are critical to the proliferation/survival of certain cell types. In screening over 30 drugs of various structures (identified by the Drug Screening Program, DTP), we have found that only certain ansamycin derivatives are capable of interacting with hsp9O.